The rapid rise in atmospheric CO2 levels presents a significant long-term threat to human health.(1) Much of the current rise in CO2 levels may be ascribed to the use of non-renewable fuels and, as a result, there has been great interest in studying energy conversion reactions such as H+ reduction and H2 oxidation.(2) Some of the best H2-producing and -consuming catalysts are [FeFe] hydrogenase enzymes which are able to operate near the thermodynamic potential of the reaction at high turnover frequencies.(3-6) Because understanding the chemical basis for their high efficiency may aid in the design of new synthetic catalysts for H2 production and consumption,(7) much attention has been given to the structure of the active site of the enzyme (the H- cluster).(8, 9) The H-cluster has a biologically unprecedented structure with a 2Fe core ligated by three CO and two CN- ligands, each of which are typically toxic in their free states. Although it is known that the CO and CN- ligands derive from tyrosine,(10-13) the mechanistic details of their formation and assembly into the H- cluster are scant. In addition, the Fe centers are bridged by a five-atom dithiolate bridge; the identity of the central atom-currently thought to be N-has been the subject of much debate, and is important because it is thought to serve as a pendant base that can kinetically facilitate H+ migration.(9, 14-20) Owing to the unusual structure of the H-cluster and its central importance in affecting the rate and redox potential of the featured H2 chemistry, the mechanism of its biosynthesis is of high interest. In this proposed work, I aim to elucidate several aspects of the mechanism by which the set of maturase enzymes HydE, HydF, and HydG promote H- cluster formation. I will focus on three questions: what are the geometric and electronic structures of Fe- containing intermediates in the early stages of cluster maturation, what are the molecular precursors that give rise to the dithiolate bridge, and can the central atom be identified directly by EPR spectroscopy? To address these questions I will perform advanced EPR experiments on specifically labeled isotopologs of both intermediates in H-cluster synthesis as well as the mature H-cluster. These isotopic labels will be introduced through the use of labeled substrates (most often tyrosine) thereby allowing for a specific isotopic label to be traced from the molecular precursor through intermediates and finally into the mature H-cluster. For each intermediate, orientation-selective EPR experiments such as HYSCORE and ENDOR will enable determination of the distances and orientations of the labeled nuclei with respect to the electron spin (modeled as a point- dipole), thereby providing detailed geometric and electronic structure information. These EPR experiments will be performed in conjunction with complimentary stopped-flow FTIR and Mossbauer spectroscopies. Overall, this work will address the mechanistic details of tyrosine degredation into CO and CN- by HydG, the structures of Fe-containing intermediates in this process, the identities of the final product(s) o the HydG reaction, and the molecular precursors to the dithiolate bridge.